Cassandra data model

Cassandra's data model is very different from the one we typically see in RDBMS. /b10> This chapter provides an overview of how Cassandra stores data.

Cluster

Cassandra databases are distributed across several machines that operate together. /b20> The outer outerst container is called a cluster. b20> For troubleshooting, each node contains a copy that, in the event of a failure, is replicated. /b20> Cassandra arranges nodes in a cluster in a circular format and assigns data to them.

Key space （Keyspace）

Keyspace is the outermost container of data in Cassandra. The basic property of a keyspace in Cassandra is -

• Replication Factor - It is the number of computers in the cluster that will receive copies of the same data.

• Replica placement policy - It's just a policy to put a copy on media. /b20> We have simple policies (rack-aware policies), old network topology policies (rack-aware policies), and network topology policies (data center sharing policies).

• Column family - The keyspace is a container for a list of one or more column familyes. /b20> A column family is also a container for a collection of rows. /b21> Each row contains a sequence. /b22> Column familyes represent the structure of the data. /b23> Each key space has at least one, usually many column families.

The syntax for creating a key space is as follows -

CREATE KEYSPACE Keyspace name
WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 3};

The following illustration shows a diagram of the key space.

Column family

A column family is a container for an ordered collection of rows. /b20> Each row is an ordered collection of columns. /b20> The following table lists the key points for distinguishing column series from relationship database tables.

Cassandra column family has the following properties -

• keys_cached - It represents the number of locations where each SSTable remains cached.
  

• rows_cached - The number of rows whose entire contents will be cached in memory.

• preload_row_cache - It specifies whether you want to prefill the row cache.

Note - Unlike the relationship table for patterns that are not fixed column familyes, Cassandra does not force a single row to own all columns.

The following illustration shows an example of a Cassandra column family.

Column

Super column

The super column is a special column, so it is also a key value pair. /b10> But the super column stores the map of the child column.

Typically, column familyes are stored in a single file on disk. /b20> Therefore, in order to optimize performance, it is important to keep the columns that you might query together in the same column family, and super columns can be helpful here. H ere's the structure of the super column.

Cassandra and RDBMS data models

The following table lists the key points for distinguishing between Cassandra's data model and RDBMS's data model.